In conventional relay networks, data packets are transmitted from a source node to a destination node via a single path with, perhaps, multiple serial hops through relay nodes.
In a cooperative relay network, wireless nodes cooperate with each other in transmitting data packets in parallel. By exploiting the broadcast nature of a wireless channel to reach multiple relay nodes simultaneously, and by enabling the relay nodes to cooperate, it is possible to reduce energy consumption in delivering a packet from the source to the destination. This can also significantly increase gains in overall throughput and energy efficiency, A. Nosratinia, T. Hunter, and A. Hedayat, “Cooperative communication in wireless networks,” IEEE Communications Magazine, vol. 42, pp. 68-73, 2004; A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity-Part I: System description,” IEEE Transactions on Communications, vol. 51, pp. 1927-1938, 2003; and J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative diversity in wireless networks: Efficient protocols and outage behavior,” IEEE Transactions on Information Theory, vol. 50, pp. 3062-3080, 2004.
In cooperative relay networks, where the nodes are powered by batteries, minimizing energy consumption is important. At the same time, it is required that outage, which occurs due to deep channel fading, is kept below a specified level.
Typically, the source node transmits data packets to the destination node in parallel using several intermediate relay nodes, A. Wittneben, I. Hammerstroem, and M. Kuhn, “Joint cooperative diversity and scheduling in low mobility wireless networks,” IEEE Global Telecommunications Conference (Globecom), vol. 2, pp. 780-784, 2004; A. E. Khandani, J. Abounadi, E. Modiano, and L. Zheng, “Cooperative routing in wireless networks,” Allerton Conference on Communications, Control and Computing, 2003; and B. Rankov and A. Wittneben, “Distributed spatial multiplexing in a wireless network,” The Asilomar Conference on Signals, Systems, and Computers, pp. 1932-1937, 2004.
Cooperative beamforming has also been referred to as distributed beamforming, see G. Barriac, R. Mudumbai, and U. Madhow, “Distributed Beamforming for Information Transfer in Sensor Networks,” IPSN 2004, pp. 81-88, 2004. Mechanisms for enabling synchronization between relays using a trigger pulse mechanism from a master relay node were described. The effect of coordination error was analyzed. However, they do not take any relay selection or outage into account. The overall energy consumption from the source to destination is also not considered.
Four simple relay selection criteria are described by J. Luo, R. S. Blum, L. J. Cimini, L. J. Greenstein, and A. M. Haimovich, “Link-Failure Probabilities for Practical Cooperative Relay Networks,” IEEE Globecom, 2005. Two of the criteria, ‘Pre-Select One Relay’ and ‘Best-Select Relay,’ select a single best relay based on a mean channel gains, while in the remaining two criteria, ‘Simple Relay’ and ‘ST-Coded Relay,’ all of the relays that decode data from the source are selected. In ‘Simple Relay,’ the relay nodes do not synchronize their phase, while in ‘ST-Coded Relay,’ a distributed space-time code is used. Hybrids of the above schemes were also described.
Search algorithms for selecting a single relay node based on an average distance or path loss between the nodes, frame error probability and pairwise codeword error probability were described by Z. Lin and E. Erkip, “Relay Search Algorithms for Coded Cooperative Systems,” IEEE Globecom, 2005.
Khandani et al. describe a model that is restricted to additive white Gaussian noise channels with phase compensation. That model does not consider dynamic fading-induced channel variations, outage, or the overhead required for cooperation between relay nodes.
Knowledge of the channel state information (CSI) at a transmitter is assumed by Laneman et al. above. However, they do not consider the cost of acquiring the CSI. Wittneben et al. only considers amplify-and-forward, which also neglects the cost of acquiring the CSI, see also M. M. Abdallah and H. C. Papadopoulos, “Beamforming algorithms for decode-and-forward relaying in wireless networks,” Conference on Information Sciences and Systems, 2005.
Because the cost of acquiring the CSI can be a significant factor in cooperative relay networks, it is desired to consider the CSI cost so that relay selection can be optimized, and total power consumption in the network can be minimized. A simple relay selection rule that selects only one relay according to a metric that is based on both the source-to-relay and relay-to-destination links was described by A. Bletsas, A. Khisti, D. P. Reed, and A. Lippman, “A simple cooperative diversity method based on network path selection,” To appear in IEEE Journal on Selected Areas in Communication, Special Issue on 4G Wireless Systems, 2006.